The present invention relates to a document feeder that separates originals stacked on a stacker and feeds an original one at a time to a reading platen, sequentially reads images on the original at the platen and stores read originals in a discharge stacker.
Generally, an original conveyor apparatus equipped on a glass platen of an image reading apparatus, such as a copier or scanner, is well known in the prior art. Such apparatuses are known to separate originals stacked on a feeder tray into single sheets and to feed a separated original to a reading position on the platen of the image reading apparatus. Then, after images on the original are read using reading means of a light source of the image reading apparatus, mirrors, a lens and CCD, such apparatuses discharge the read original to a storage tray.
Such apparatuses often arrange a sheet supply stacker and a discharge stacker one above the other over a platen, and convey an original over a platen from the sheet supply stacker to the discharge stacker through a U-shaped path. Accordingly, such apparatuses have a feed path at an upstream side of the platen, and a discharge path at a downstream side of the platen. Generally, these paths are formed into a U-shape because of the vertical arrangement of stackers. One or a plurality of roller pairs is arranged in the feed path for conveying an original to the platen. Similarly, the discharge path is also arranged with one or a plurality of roller pairs for discharging an original from the platen to the discharge stacker. These roller pairs are configured to be driven by a drive motor. The system is controlled to convey an original at a predetermined, constant conveying speed up to a predetermined position on the platen, and then switch to a reading speed that is set according to whether the original is in color or in black and white, and according to a reading resolution, in order to convey the original at that reading speed. Then, the system is controlled to discharge the original.
Like the system disclosed in the Japanese laid-open patent Publication No. 2001-302053, these kinds of apparatuses are controlled to stop an original after reading is completed, and then to kick out a next original on the sheet supply stacker by the reverse drive of a drive motor. The drive motor will then switch from the reverse drive to a forward drive when the next original reaches a registration position disposed partway in the feed path for conveying an original from the sheet supply stacker to the platen. This action simultaneously conveys the next original to the platen and conveys the previous original to the discharge stacker.
However, in recent years, demand has increased for an apparatus that can read originals, for high quality images that are free of distortion or image skewing, and that can handle large volumes of originals. Thus, to attain good quality readings of originals, it is preferred to reduce the angle for lifting an original in order to prevent shaking of original images that is caused by the shock of lifting the original from the platen. Conversely, to increase capacity of a discharge stacker, the discharge stacker must be made deeper to hold larger volumes of originals. In order to keep the lifting angle of the original small, and to enable a discharge stacker to hold a large volume of originals, the discharge path from a reading position to the discharge stacker naturally must be longer in comparison to the feed path.
Furthermore, applying the conveyance control method described above to this apparatus would cause an original to be discharged to the discharge stacker while a next original is being read by reading means. This causes the problem of varying the load on the drive by discharging an original, and can degrade the image quality of a read image. Still further, a previous original would thus be discharged to the discharge stacker at the same speed as the reading speed of a next original. If the original is discharged at a high speed, the originals on the discharge stacker would become scattered on the discharge stacker. Conversely, discharging at a slow speed would allow the trailing edge of an original to remain at the discharge roller position, thereby causing a problem of improperly aligned discharged originals.
In order to solve these problems, Japanese laid-open patent publication Heisei 11-222345 discloses controlling a feeder to convey originals at a constant rate, with a minimum gap therebetween using a speed of a predetermined range where the reading speed does not hinder the alignment of discharged originals. If the reading speed is outside a predetermined range, the system is controlled to feed a next original after switching the speed to within a predetermined range that also does not hinder the alignment of discharging the previous originals before feeding a next original.
However, even if the means described above are employed, the alignment of discharged originals is not good because the reading speed is within a predetermined range, but different from the discharging speed. Also, if the reading speed is not within a predetermined range, a previous original will be discharged to the discharge stacker while a next original is being read by the reading means. The foregoing varies the load on the drive by discharging an original which degrades the image quality of a read image. Thus, the aforementioned problems remain unsolved. Furthermore, if the reading speed is outside a predetermined speed, the original is discharged after switching to a discharging speed before starting to convey a next original. This means that original processing time increases, which is not desired.